1. Field of the Invention
The present invention relates to a rotating electric machine capable of suppressing torque pulsation and a method of manufacturing the rotating electric machine and particularly to split cores mainly used for improvement in motor efficiency or productivity.
2. Description of the Related Art
Split cores are formed by punching steel sheets with a press mold to prepare split core members, stacking the plurality of split core members, fixing the stacked split core members by caulking, as disclosed in Patent Document 1 [Japanese Patent No. 3461552 (third page and FIG. 1)]. A stator core is prepared by winding a coil around magnetic pole teeth (magnetic tooth portions) of the split cores with insulation sheets interposed therebetween, assembling the coiled split cores in an annular shape, and fixing the split cores assembled in the annular shape within a frame.
Since the coil is wound in a state where the split cores are separated, the coil can be wound with high density without a waste of a space through which a nozzle winding the coil passes, compared to a coil wound around an integrated core which is not divided. Accordingly, it is possible to improve torque thanks to an increase in the number of wound coils or to reduce the electroresistance caused upon mounting coils having a larger cross-section area, thereby improving motor torque or performance such as efficiency.
There has been increased a demand for reducing torque pulsation in order to improve accuracy or comfortableness in a driving motor used for a machine tool, an electrically-driven power steering of a passenger car, and an elevator, for example. The torque pulsation is caused by various factors such as an error in an inner circumferential shape of a stator core, a declination of magnetoresistance of a stator core, a declination of a magnetomotive force of a rotor, and a shaft deviation between a stator and a rotor.
When the annular stator core is obtained by assembling the split cores, boundaries of the split cores come in contact with each other to determine the respective positions of the split cores one another. However, a small stepped portion occurs in the inner circumferential shape of the split cores due to an error in process accuracy or assembly of the split cores. When the small stepped portion exists in the inner circumferential shape of the split cores, permeance of a magnetic path formed between a stator and a rotor is locally increased. Therefore, a problem occurs in that the torque pulsation increases.
In order to improve the assembly accuracy of the split cores, there were carried out studies in which positioning concave and convex portions are formed in a contact portion with the adjacent split cores in a radial direction or the inner circumference is pushed against a cylindrical spindle to assemble the split cores. However, the assembly accuracy determined by the process accuracy such as a punching error of split core members is not sufficient to suppress the torque pulsation. Moreover, when a request for the reduction of the torque pulsation is strict, a machining accuracy has to be further improved or grinding and finishing on the inner circumference are necessary after the assembly. Therefore, a problem occurs in that manufacture cost increases.
A steel sheet has magnetic anisotropy in which magnetoresistance is different in a rolling direction and a direction perpendicular to the rolling direction. Therefore, when the split cores are arranged to be integrally punched, the magnetoresistance is not uniform according to a direction of magnetic pole tooth portions. For that reason, a problem occurs in that the torque pulsation increases even though there is no error caused in processing and assembling.
The torque pulsation occurring due to the shape of a stator core or a material characteristic is caused by magnetic asymmetry in which the magnetoresistance between a stator core and a rotor core opposite the stator core is distributed unevenly in a stator circumferential direction.